A wide variety of metal implants are used clinically within the human body. These range from permanently implanted metals such as orthopedic replacements, coronary arterial stents, or brachytherapy seeds, and include temporarily implanted metals such as surgical staples.
Hypodermic needles are one such temporary metal implant used for a wide variety of clinical applications. Metal needles are commonly used for localized drug delivery, such as in cancer therapy, or for tissue collection in biopsy. In either case, the ability to visualize both the anatomical surrounding structures and the advancing needle tip is required. Several procedures, such as drug delivery, tissue biopsy collection, or brachytherapy seed placement, all require accurate injection of a metal needle. Needle tracking also requires the visualization of the tissue background so that specific tissue landmarks such as tumors or lesions may be targeted or avoided when guiding the needle into position. Needle deflection and deformation can occur when inserting needles into soft, non-homogeneous tissues, which can affect the localized accuracy of insertion.
Currently, the most commonly used clinical strategy to visualize the direction of the needle shaft in real time is ultrasound guidance. However, the needle tip can often be visualized better than the needle shaft because of the irregular surface of the machine-cut bevel, which scatters the ultrasound (US) beam in all directions, reflecting the beam, in part, back to the transducer. However, visibility of the tip alone is not sufficient for the clinician to gauge the insertion angle of the needle. Visibility of the needle shaft is dependent on the angle of the needle relative to the transducer and is best visualized only when perpendicular and in the plane of the US transducer. Needle deflections away from the transducer of only a few degrees are usually enough to conceal the US signal from the needle. To overcome these issues, mechanical or optical needle guides are often used to keep the needle in the transducer plane. However, these guides restrict needle movement when fine adjustments are needed by medical operators; therefore, many clinicians prefer using a freehand technique during needle insertion and injection.
US imaging of metal needles also produces an unexpected echographic pattern resembling the shape of a comet tail. The pattern is due to reverberations within the needle and is dependent on the acoustic impedance mismatch between the needle and its surroundings. In fact, these artifacts are present in US imaging of any metallic object. While these artifacts can be helpful in determining the presence of foreign metal in vivo, the existence of the comet tail pattern prevents visibility of objects directly distal and adjacent to the needle (relative to the position of the transducer). This effect can detrimentally affect the ability to determine needle position relative to the background tissue.
Likewise, when using current ultrasound technology to track needle insertions/interventions, tissue background has been determined through the ultrasound speckle shown on the imaging screen. However, the acoustic speckle between different tissue compositions does not always offer enough contrast in order to determine what tissue landmarks the needle tip has penetrated to. Though photoacoustic imaging can utilize optical absorption to differentiate tissue types, some tissues have very low absorption and therefore may have very low signals when imaged under photoacoustic imaging. The difference in acoustic scattering between healthy and diseased tissue is not usually very high. Therefore, needle trajectories can oftentimes miss the region of interest that the operator is interested in when basing decisions on ultrasound alone. Furthermore, when targeting needle interventions into soft tissue, the real-time imaging modality should confirm the arrival of the needle into the region of interest. Without confirmation of needle arrival, false negatives can occur when conducting biopsies.
Current technologies only allow for needle tracking with limited to no tissue characterization. In ultrasound, only landmarks that contain strong differences in acoustic scattering can be differentiated. Similarly, in photoacoustic imaging alone, only tissue with strong optical absorption can be visualized. For example, fat is not easily differentiated from muscle in ultrasound, nor does it have a high absorption for visualization in photoacoustics.
In addition to temporary metal implants, permanent metal implants, such as coronary arterial stents, are also used clinically within the human body. Coronary stents are currently the most widely used coronary intervention in the United States. While the procedure is more than 95% successful, stents have brought along several unique issues including restenosis, hyperplasia and stent drift. The ability to visualize stents both during the stenting procedure and during post-surgery follow-up is important in order to correctly assess the stent with respect to the plaques and vessel, and also identify its apposition within the vessel wall.
Immediately following a stenting procedure, it is important to determine the relation of the stent struts to the vessel wall. Ideally, the stent is deployed in contact with the lumen wall; however, malapposition can occur resulting in the stent detaching itself from the wall. This detachment can cause turbulent eddies to form in the vessel which can lead to thrombosis in the area of the stent. It is important when monitoring the stent to determine how much restenosis has formed around the stent struts. The distance that the stent struts are embedded into the vessel wall must be determined to assess stent viability.
Currently, the most common method for assessing stent position is x-ray coronary angiography/fluoroscopy. However, this procedure is problematic due to its use of ionizing radiation and possible complications in using iodinated contrast agents. Furthermore, x-ray fluoroscopy can only depict a two-dimensional projection which can lead to an underestimation of lumen diameter and the stent apposition within the lumen.
Magnetic resonance imaging has been used to image stents due to its avoidance of radiation exposure and iodine contrast agents; however, the metallic composition of stents can cause susceptibility artifacts which can obscure the stent lumen and make it very difficult to visualize the relation between the stent and the vessel wall. Long scan times and low resolution also remain a major limitation. Multi-slice computed tomographic angiography (MSCTA) has been shown to image much faster than MRI; however, its low resolution and artifacts in metallic stents make assessing the surrounding vessel difficult.
Both intravascular ultrasound (IVUS) and optical coherence tomography (OCT) have reached widespread usage in catheterization labs. IVUS can detect signal reflections from the stent struts, but has insufficient contrast to determine the struts' position against the vessel wall. Ultrasound contrast of stents is affected by the background tissue environment, which is also acoustically scattering. In addition, extraneous beams of ultrasound generated by the ultrasound transmit pulse and then scattered by the metallic stents will obscure the edges of the stent borders. These blurred edges are image artifacts that can reduce the spatial registration of the imaged stent. OCT directly competes with these disadvantages with a resolution of 10-20 μm but has severe depth limitations, allowing only a penetration depth of about 2 mm. The presence of blood flowing through the vessel limits this depth even further, requiring clinicians to flush the vessel during the imaging procedure. Furthermore, the tissue behind the stent strut becomes hidden due to scattering shadows in OCT, which prevents complete diagnosis of the stent's relation to the vessel lumen.